Gas generating compositions are extremely useful in the automotive passive restraint (air bag) industry, although other uses, such as commercial or military aircraft applications, are contemplated for such gas generating compositions. Today, most, if not all, new automobiles are equipped with single or multiple air bags to protect the driver and passengers. In the near future it is expected that aircraft manufacturers will be under similar government mandates. In the operation of air bags, sufficient gas must be generated to inflate the device in a fraction of a second. The air bag must fully inflate between the time that the automobile is impacted in a collision, and the time the driver or passenger would otherwise be thrust forward against the steering wheel, dashboard or sideways against the door of a vehicle or aircraft. Consequently, nearly instantaneous gas generation is required.
There are a number of mandated design specifications required by automobile manufacturers and other regulator agencies that must be adhered to in the preparation of gas generating compositions. One such required specification is that the composition produces gas at a specific rate. Automobile manufacturers require that the gas be generated at a sufficiently and reasonably low temperature so that the occupants of the involved automobile are not burned upon impacting an inflated air bag. Inconsistent ballistic output is a major problem with all pyrotechnic inflators. Accordingly, a need exists for a formulation that minimizes the ballistic variability, prevents the production of excessive heat and maintains an adequate burn rate while generating the cleanest possible gas required to fill the specified air bag's internal envelope.
Another specified requirement of the automobile manufacturing industry is that gas generating compositions strictly limit the generation of toxic gases or solids such as but not limited to carbon monoxide, carbon dioxide, nitrogen oxide, sulfur oxide, and hydrogen sulfide. Another related design requirement is that the gas generant composition produces a limited quantity of particulate materials, which can interfere with the operation of the passive restraint system, create an inhalation hazard, irritate the skin and eyes, or present a hazardous solid waste that must be disposed of in an environmentally safe manner.
Sodium azide is one such hazardous constituent of gas generating compositions that is currently being phased out by the industry due to its high toxicity as taught in U.S. Pat. No. 6,661,261 to Ramaswamy, et al. and U.S. Pat. No. 5,516,377 to Highsmith, et al. Further, the use of sodium azide (or other azides) results in extra expense and risk in manufacture of gas generant due to the extreme toxicity of azides.
It has also been found that the non-azide propellant technologies are costly to manufacture, and have inherent performance problems such as high burn temperature, undesirable trace effluent values and inconsistent ballistic output. High burn temperatures are undesirable because the gas requires more cooling to maintain acceptable gas temperatures. Cooling of the gas is typically performed by the inflator filtration system. A disadvantage of the filter is that it also increases material costs. On the other hand, the baffle system minimizes cost. Cool gas temperatures, however, are required to prevent the air bag and subsequently the occupant of the automobile or aircraft from burning.
It would be preferable, therefore, to have a gas generating composition that produces more gas and fewer solids. The nongaseous fraction of the gas generant products must be contained or filtered to provide a clean inflating gas. It would also be desirable that when the composition produces particulates, the majority of these particulates are filterable, solid slag. When conventional gas generating compositions that included a combination of 5AT and a metal oxide were detonated, it was observed that significant quantities of slag formed as a byproduct. The term slag is herein defined as insoluble metallic particulate. This slag was generated in addition to the gasses generated by the ignition of the gas generating composition containing the metal oxide. Slag can be easily filtered, preventing the airborne reaction products from escaping into the surrounding environment during and after air bag deployment. Filtration, therefore, serves a function that limits the dissipation of potentially harmful dust in the vicinity of the spent air bag, which could otherwise cause secondary effects to the passengers and others in the vicinity such as eye, lung, and mucous membrane irritation.
Currently available 5AT based gas generating compositions form a minimum of water-soluble products at combustion of the gas generating composition form a minimum of water-soluble products at combustion of the gas generant. For example, U.S. Pat. No. 5,500,059 to Lund, et al. teaches that copper oxide is a preferred oxidizer in SAT based gas generating compositions. U.S. Pat. No. 5,139,588 to Poole teaches the use of transition metal oxides and other metal oxides having high melting points in 5AT based gas generating compositions to function as high temperature slag forming material.
The utilization of waxes in ballistic formulation is taught in several U.S. patent references. Examples include U.S. Pat. No. 4,315,787 to Hattori, which suggests the use of wax or oil to create a separate, stable phase in a water mixture. The two phases of the emulsion are combined upon impact, causing detonation. In Hattori '787, the wax is taught to be a combustible substrate at 1% to 7% of the compound. The U.S. Pat. No. 4,394,198 to Takeuchi also teaches the use of an oil or wax material as a combustible element in a two-phase mixture. In Takeuchi '198 the wax or oil is taught to amount to 1% to 10% of the total composition. The U.S. Pat. No. 4,500,369 to Tag discloses another variation on the water-in-oil explosive mixture, where 2% to 6% of wax is added.
For other than two-phase explosive mixtures, the U.S. Pat. No. 4,736,638 to Bachman teaches the addition of a high molecular weight polymer to an ANFO (ammonium nitrate and fuel oil) explosive mixture. A small amount (approximately 0.1%) of the polymer is added to the ANFO to change the viscosity and detonation properties of the mixture. The U.S. Pat. No. 5,597,977 Chattopadhyay discloses the hardening of ammonium nitrate (AN) with a polymer additive, such as polystyrene. Chattopadhyay '977 teaches the addition of up to 10% of the polymer. Chattopadhyay '977 directs his application toward the hardening of the AN. The U.S. Pat. No. 5,641,938 to Holland teaches a gas generating composition that includes an elastomeric binder. Holland '938 specifically directs his invention to non-azide air bag inflation applications. However, the binders disclosed are only plasticizers that comprise up to about 10% of the total mixture. Holland '938 teaches the use of nitroguanidine as a primary fuel.
The primary purpose of the waxes and plastics, especially in water free mixtures, relates to the binding properties that smaller quantities of the plastic materials impart to the mixture.